1. Technical Field
Described herein are a semiconductor device and a method for manufacturing the semiconductor device.
2. Related Art
Since flash memories that are nonvolatile memories, particularly NAND flash memories, are easily made smaller in size, the prices of those flash memories are rapidly declining, and the capacities of those memories are rapidly becoming larger. Having high shock resistance, those memories are being rapidly developed as still image memory media and high-quality voice recording media, and are now forming a large market.
The memory cells used in NAND flash memories are characterized by being of a floating gate (FG) type having a floating gate that is covered with an insulating film and is made of polysilicon, a MONOS (Metal-Oxide-Nitride-Oxide-Silicon) type having a charge trap film that is covered with an insulating film and is formed with a silicon nitride film, or a SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) type. A control gate that is to be the charge storage film is formed on the floating gate or the charge trap film, and sandwiches an interelectrode insulating film or a block insulating film. The voltage (control voltage) to be applied to the control gate is controlled. Also, electrons are injected (writing) by FN (Fowler-Nordheim) tunneling into the floating gate or the charge trap film from the substrate via the tunnel insulating film, or electrons are withdrawn (erasing in a FG type or a MONOS/SONOS type) from the floating gate via the tunnel insulating film, or holes are injected into the charge trap film so as to cause electron-pair annihilation (auxiliary erasing in a MONOS/SONOS type). In this manner, the threshold values of memory cells are varied. However, as the sizes of the memories have become smaller, one large problem has surfaced.
To increase the memory capacities, reductions in device size are most effective. However, it is also necessary to reduce the charge storage film in size. Even in a case where the charge storage film is made smaller, the charge amount required for driving the memory cells should be as high as possible, so as to prevent errors and realize multivalue recording to increase the number of bits stored in each cell. In other words, it is necessary to improve the charge capture density. Moreover, if the size in the horizontal direction (the direction perpendicular to the depositing direction of the charge storage film) becomes smaller, the size in the vertical direction (the depositing direction of the charge storage film) also becomes smaller, or the film thickness becomes smaller. As a result, the capture time becomes shorter, and the amount of released charges becomes larger. Therefore, it is necessary to improve the capture efficiency and the retention characteristics.
To improve the charge capture density, a method for improving the silicon concentration in the silicon nitride film to be the charge storage film has been suggested (see JP-A 2003-347543 (KOKAI), for example). According to the technique disclosed in JP-A 2003-347543 (KOKAI), however, a silicon nitride film is deposited directly on the tunnel insulating film through a heating process. As a result, the tunnel insulating film is nitrided, and a charge trap source is formed. Since charges are captured at the portion where the trap source is generated, charges are not efficiently injected into the charge storage film, and the capture efficiency becomes poorer.
Also, a technique has been suggested to maintain high retention characteristics by forming a block insulating film with a high-dielectric film (high-k film) provided above the charge storage film. In this manner, higher insulating properties are achieved, and captured charges are not released (see JP-A 2007-287856 (KOKAI)). According to the technique disclosed in JP-A 2007-287856 (KOKAI), however, the block insulating film containing oxygen and metal is deposited directly on the silicon nitride film to be the charge storage film. Therefore, oxygen is diffused into the silicon nitride film, and defects are corrected through oxidation. As a result, the capture efficiency becomes poorer.